The retreat of the ice covering “Snowball Earth” 700 million years ago might have been the key to the Cambrian explosion that seeded our planet with diverse forms of life. But the trigger may not have been the changes to the climate, but rather the release of phosphorus into the ocean.

During this time period, called the Cryogenian or Snowball Earth stage, the entire planet was covered in snow and ice, and the oceans may even have been frozen. Many researchers believe that the ice receded twice during this freezing period, first around 700 million years ago and then again around 635 million years ago. In a paper published in Nature this week, a team of researchers propose that these receding sheets released phosphorus into the oceans.

In the scheme offered by [Noah] Planavsky and his colleagues, the snowball ice sheets would, as their modern counterparts do, grind up continental rock that would release phosphorus when the glaciers retreated. That phosphorus would wash into the ocean, where it would fertilize algal blooms that could drive a surge in the production of organic matter and oxygen. And the added organic matter that settled into the mud on the ocean bottom would leave additional oxygen behind, eventually boosting atmospheric and oceanic oxygen. [ScienceNOW]

Phosphorus isn’t usually present in the oceans in high concentrations, but it is essential, so it is often the piece missing from the algal growth equation. To figure out if phosphorus levels rose before the Cambrian explosion, the researchers, lead by Planavsky, studied marine core samples from around the world.

“Gathering samples for this study was a long endeavor,” recalled researcher Noah Planavsky, a biogeochemist at California-Riverside. “Cars will get stuck in mud, flat tires are unavoidable, and old maps can lead you wandering for hours. It is essential to accept that you could spend days traveling to sample some rocks, sometimes halfway across the world, and find nothing useful for your study.” [LiveScience]

What they got from this world-trudging was evidence for a spike in phosphorus levels in the oceans that occurred between 635 and 750 million years ago, about the time of the recession of Snowball Earth’s ice sheets.

“We have potentially found the smoking gun for the mechanism that drove the oxygen increase that ushered in the first animals,” said researcher Timothy Lyons, another biogeochemist at California-Riverside. “Our results may be the first to capture the nutrient driver that was behind this major step in the history of life, and that driver was ultimately tied to the extreme climate of the period.” [LiveScience]

Some researchers disagree with this model, because phosphorus has a limited lifetime in the ocean, and because some believe there was only a single thawing of Snowball Earth. In order to have a strong impact on ocean life, critics say, the levels of phosphorus in the ocean would need to be elevated for longer than could be attributed to a single thaw. But Planavsky argues that even if there was just one thawing event, ice sheets would have moved back and forth repeatedly during the warming, and phosphorus would have been released continually over millions of years. That would have kept the phosphorus levels high, he says, spurring life in the oceans.

Other researchers are interested, but not yet convinced.

A phosphorus link between glaciations and oxygen, and therefore evolution, “is a fascinating possibility,” says biogeochemist Donald Canfield of the University of Southern Denmark in Odense. “But they don’t yet have the continuous [geologic] record that would prove it. In principle, it’s a testable hypothesis; that’s the work’s value.” [ScienceNOW]

Leaving aside did or did not melting of snowball earth fertilize the oceans with phosphorus, why would the algal blooms caused by this boost oceanic oxygen? Algal blooms today caused by fertilizer runoff carried by rivers into the ocean are blamed for causing dead zones. The ferilizaztion causes an algal bloom, the algy dies and sinks into the ocean, the dead algy decays consuming oxygen, this causes an area of reduced oxygen disolved in the ocean, adead zone in which fish cannot survive because the disolved oxygen is too low. How could algal blooms then increase disolved oxygen, but now decrease disolved oxygen?

scott

All for doing everything we can do to reduce and get rid of human induced pollutants…but say this was starting up again, or even a more recent style ice age was showing signs of getting fired up…would we be pumping as much crud as we could into the air to keep the planet warm?

Damian

Judging from the image accompanying this post, you appear to be a plate tectonics denialist!

Jennifer Welsh

Hi all!

@TRJc, I’m no dead zone expert, but off the top of my head my guess would be that back then the atmosphere was very different (extremely lacking in oxygen), and so the algeal bloom would have reacted very differently to phosphorus than they are today in the dead zone in the Gulf. From the study’s abstract:

“An enhanced postglacial phosphate flux would have caused high rates of primary productivity and organic carbon burial and a transition to more oxidizing conditions in the ocean and atmosphere…. We propose that these two factors are intimately linked; a glacially induced nutrient surplus could have led to an increase in atmospheric oxygen, paving the way for the rise of metazoan life.”

Anyone have any other ideas?

@Damian, I was wondering if someone would notice! I realized that when I put the post together, but it was the best picture I could find that we could use. I swear to you, I am NOT a plate tectonics denialist. I’m also not a witch.

Thanks for reading and commenting!

Jen

John

Considering the large time gap between this development and the Cambrian Explosion which onset some 80 million years later terms like “smoking gun” and “trigger” seem a bit incongruous.

Chris

@TRJc and Jennifer:

You’re quite right that the algal blooms associated with fertilizer runoff consume oxygen in the water column. But some portion of the organic matter produced makes it through to the sediments, and becomes buried into the long-term carbon cycle. That portion of organic matter that is removed releases a stoichiometrically equivalent amount of oxygen to the ocean/atmosphere system.

You can think of it this way – photosynthesis generates organic matter and oxygen. Respiration runs the reaction in reverse, consuming both (this is why oxygen in the water column is consumed as organic matter settles). If all the organic matter produced during photosynthesis was consumed in this way, there would be no oxygen left to accumulate in the atmosphere. But a little bit escapes into sediments, releasing oxygen. In fact, many would argue that this burial process becomes more efficient if oxygen in the water column is consumed. So, in the simplest sense, the counterintuitive result of increasing photosynthetic production is that while water column oxygen concentrations decrease, there is ultimately more oxygen released into the atmosphere on longer timescales.

@Damian:

You’re right that there is a bit of a timescale mismatch! The paper doesn’t really make any links to the ‘Cambrian Explosion’, which, as you point out, occurred much later than the Cryogenian glacials. But there has emerged a pretty rich record of metazoan (animal) evolution prior to the Cambrian, and it appears to have really gotten going following these glacial events. This may or may not be the right mechanism, but there’s definitely a temporal relationship between the glacials and the initial expansion of metazoan life.